Electronic devices including digital electronic devices typically represented by personal computers, game machines, or portable information terminals are promoted to undergo high densification of circuits, in parallel with the adaptation of circuits to high frequencies and the enhancement of circuit performance; thus passive elements tend to be affected by active elements such as semiconductors which radiate noises. Conventionally, as the countermeasures against these noises, ferrite core and electromagnetic wave absorbers for quasimicrowave band have been used; however, downsizing of electronic appliances comes to demand for the components as countermeasures against noises to be reduced in size and thickness and to be enhanced in performance.
For the purpose of meeting the EMC standard, it comes to be a critical challenge to meet the standard associated with noises around 100 to 400 MHz, and accordingly demand for electromagnetic wave absorbers and minute components as countermeasures against EMI, corresponding to this band, is growing. Japanese Patent Laid-Open No. 2000-4097 discloses a method for manufacturing a composite magnetic sheet that can attain high permeability for the frequencies of 100 MHz or less in which the resonance frequency is made to be reduced in such a way that a flaky magnetic powder is annealed to reduce the residual stress, and thereafter oriented along the in-plane direction and pressed perpendicularly to the sheet plane at a temperature of the glass transition temperature Tg of the organic bonding agent involved or higher. However, the permeability of such a composite magnetic sheet made of an organic bonding agent and a flaky magnetic powder is at most 20 at 100 MHz, so that high permeability is hardly obtainable.
Japanese Patent Laid-Open No. 11-74140 discloses a method for manufacturing a dust core, the method being characterized in that a flaky soft magnetic powder is used and the core is molded into a plate-like form by extrusion molding. The method has an advantage that the permeability can be made high because the flaky soft magnetic powder is oriented along the extrusion direction; however, when a sheet thinner than 0.4 mm is formed, the sheet is needed to be taken over by applying tension and to be made thinner immediately after the sheet has been extruded from a narrow nozzle, so that high permeability becomes hardly obtainable. In other words, it is necessary to lower the viscosity at the extrusion temperature by increasing the resin content so that a sufficient flexibility may be provided to take over the sheet when extruded from the narrow nozzle; accordingly, the filled content of the magnetic powder is reduced and hence high permeability cannot be obtained.
Other methods are also disclosed in which extrusion is not adopted but the print lamination method and the doctor blading are adopted to make the sheet thinner.
Japanese Patent Laid-Open No. 11-176680 discloses a method for manufacturing a core in which a 500 μm or less thick sheet is manufactured with a flaky soft magnetic metal powder from 5 to 40 of aspect ratio and a binder by means of the print lamination method, the sheets thus obtained are laminated to make a 10 mm or less thick laminate, and the laminate is subjected to press molding and subsequently to blanking to yield a core. However, even if this method is adopted, because this method uses a large amount of organic binder besides solvent, the packing density of the soft magnetic metal powder is hardly made to be higher than 75%; additionally, the stress deterioration involved in molding cannot be avoided, and a heat treatment capable of effectively relieving the residual stress cannot be applied, making it impossible to obtain high permeability for high frequencies around 100 MHz.
Japanese Patent Laid-Open No. 2000-243615 discloses a method for manufacturing a composite magnetic material in which a film is formed of a slurry-like mixture comprising a flaky soft magnetic powder, a binder and a solvent; the method is characterized in that the composite magnetic material is manufactured such that no stress strain is once again applied to the flaky soft magnetic powder from which the stress strain has been relieved; however, because the method in which no deformation stress is applied to the flaky soft magnetic powder itself has drawbacks including the fact that the method can hardly make the packing density of the material high and additionally the fact that the stress generation due to the curing and contraction of the resin cannot be avoided in principle, it cannot be expected to obtain high permeability for the high frequencies around 100 MHz.
Any of the conventional techniques is a technique based on a technological concept that puts particular emphasis on the consideration such that after the residual stress of the flaky soft magnetic metal powder has been reduced, no excessively large stress may be applied to the flaky soft magnetic metal powder in the molding step; such a technological concept involves two drawbacks that the packing density of the metal powder substantially cannot be made high and additionally the residual stress in the molded body cannot be made small, and leads to a limit in improving the complex permeability for the high frequencies ranging from several tens MHz to several GHz.
It is an object of the present invention to provide a thin (e. g., 0.4 mm or less) composite magnetic material, by overcoming the above described problems, which is excellent in the noise reduction effect in high frequencies, particularly, in the range from 100 to 400 MHz critical in meeting the EMC standard, a method for manufacturing a sheet-like product thereof, and the like. Additionally, the present invention takes as its another object to provide an electromagnetic wave absorbing sheet, by overcoming the above described problems, which has excellent permeability for high frequencies and a method for manufacturing the same.